The present invention relates to an aluminum base alloy clad material for use in heat exchangers in which corrosive liquid is employed as a heat exchange fluid, and more particularly to an aluminum base alloy clad material which is resistant to corrosion in corrosive liquids due to a sacrificial anode effect afforded by a soldering process in a non-oxidizing and reduced-pressure atmosphere when making heat exchangers, and which is also workable in order to make parts for heat exchangers.
Aluminum heat exchangers are now in use widely and the heat exchange tubes of aluminum heat exchangers are made of aluminum or aluminum base alloys. However, the use of aluminum or aluminum base alloys is limited to coolers and refrigerators whose heat exchange liquids are non-corrosive, for example, Freon made by E. I. du Pont de Nemours & Co., Inc.
This is because when the heat exchange fluid is water, for example, in which some corrosive materials are unavoidably dissolved, as in the case of radiators of cars, considerable pitting corrosion occurs in the fluid passage members, such as plates, tubes or frames, made of aluminum or aluminum base alloys, of the heat exchangers, so that there is a risk that fluid leakage accidents may occur. As a countermeasure for preventing such accidents, prevention of corrosion by utilizing sacrificial anodes is known to be effective and it is widely known that Al-Zn base alloys are useful for that purpose.
However, when heat exchangers are assembled, insomuch Al-Zn base alloys, by soldering methods in a non-oxidizing and reduced-pressure atmosphere, such as by a vacuum soldering method or a pressure-adjusted soldering method, zinc is evaporated during the soldering process and may impair the sacrificial anode effect, although such soldering methods have advantages over other methods in terms of productivity and prevention of air pollution. Therefore, it is disadvantageous to use Al-Zn base alloys for assembling heat exchangers when the above-mentioned methods are employed.
The inventors of the present invention have proposed the use of Al-Sn base alloys as materials suitable for sacrificial anodes which do not have the above-mentioned shortcomings. However, Al-Sn base alloys have problems, such low workability during the manufacture of the parts for heat exchangers due to grain boundary diffusion and occasional intergranular corrosion of Sn.